Chapter 7
Introduction to Local Area Networks
1
Chapter Seven - Introduction to Local Area Networ
Introduction
A local area network is a communication network that
interconnects a variety of data communicating devices within
a small geographic area and broadcasts data at high data
transfer rates with very low error rates.
Since the local area network first appeared in the 1970s, its
use has become widespread in commercial and academic
environments.
2
Chapter Seven - Introduction to Local Area Networ
Primary Function of a LAN
To provide access to hardware and software resources that will allow
users to perform one or more of the following activities:
File serving - A large storage disk drive acts as a central storage
repository.
For example, suppose two or more users wish to share a data
set. In
this case, the data set, like the application software, would be
stored
on the file server, while the network provided access to those
users
who had the appropriate permissions..
Print serving - Providing the authorization to access a particular printer,
accept and queue print jobs, and providing a user access to the print
3
queue to perform administrative duties.
Chapter Seven - Introduction to Local Area Networ
Primary Function of a LAN continued
Video transfers - High speed LANs are capable of supporting
video image and live video transfers.
Manufacturing support - LANs can support manufacturing
and industrial environments.
Academic support – In classrooms, labs, and wireless
E-mail support
Interconnection between multiple systems
4
Chapter Seven - Introduction to Local Area Networ
Advantages of Local Area Networks
Ability to share hardware and software resources.
Individual workstation might survive network failure.
Component and system evolution are possible (both evolutions can be
independent of each other).
• For example, if new workstations are desired, it is possible to replace older
workstations with newer ones with few, if any, changes required to the network
itself.
Support for heterogeneous forms of hardware and software.
• Under some conditions, local area networks allow equipment from different
manufacturers to be mixed on the same network. For example, IBM personal
computers with Apple microcomputers.
Private ownership.
Secure transfers at high speeds with low error rates.
5
Chapter Seven - Introduction to Local Area Networ
Advantages of Local Area Networks
Access to other LANs and WANs (Figure 7-1).
6
Chapter Seven - Introduction to Local Area Networ
Disadvantages of Local Area Networks
Equipment and support can be costly.
Level of maintenance continues to grow.
Some types of hardware may not interoperate.
Just because a LAN can support two different kinds of packages does not
mean their data can interchange easily.
• For example, even if a local area network supports two different types of
database systems, users may not be able to share data between the two database
systems.
A LAN is only as strong as it weakest link, and there are many links.
• For example, a network may suffer if the file server cannot adequately serve all
the requests from users of the network.
7
Chapter Seven - Introduction to Local Area Networ
Basic Network Topologies
Local area networks are interconnected using one of four
basic configurations:
1. Bus/tree
2. Star-wired bus
3. Star-wired ring
4. Wireless
8
Chapter Seven - Introduction to Local Area Networ
Bus/Tree Topology
The original topology
Workstation has a network interface card (NIC) that attaches
to the bus (a coaxial cable) via a tap.
Data can be transferred using either baseband digital signals
or broadband analog signals.
9
Chapter Seven - Introduction to Local Area Networ
10
Chapter Seven - Introduction to Local Area Networ
11
Chapter Seven - Introduction to Local Area Networ
Bus/Tree Topology
Baseband signals are bidirectional and more outward in both
directions from the workstation transmitting.
Broadband signals are usually uni-directional and transmit in
only one direction. Because of this, special wiring
considerations are necessary.
Buses can be split and joined, creating trees.
12
Chapter Seven - Introduction to Local Area Networ
13
Chapter Seven - Introduction to Local Area Networ
14
Chapter Seven - Introduction to Local Area Networ
Star-wired Bus Topology
Logically operates as a bus, but physically looks like a star.
Star design is based on hub. All workstations attach to a
central device such as hub.
Unshielded twisted pair usually used to connect workstation
to hub.
Hub takes incoming signal and immediately broadcasts it out
to all other workstations (or devices) connected to the
hub.
Hubs can be interconnected to extend size of network.
15
Chapter Seven - Introduction to Local Area Networ
16
Chapter Seven - Introduction to Local Area Networ
17
Chapter Seven - Introduction to Local Area Networ
Star-wired Bus Topology
Modular connectors and twisted pair make installation and
maintenance of star-wired bus better than standard bus.
• simple installation and maintenance, low-cost components
Hubs can be interconnected with twisted pair, coaxial cable,
or fiber optic cable.
Biggest disadvantage: when one station talks, everyone hears
it. This is called a shared network. All devices are sharing
the network medium.
18
Chapter Seven - Introduction to Local Area Networ
Star-wired Ring Topology
Logically operates as a ring but physically appears as a star.
Star-wired ring topology is based on MAU (multi-station
access unit) which functions similarly to a hub.
Where a hub immediately broadcasts all incoming signals
onto all connected links, the MAU passes the signal around in
a ring fashion.
Like hubs, MAUs can be interconnected to increase network
size.
19
Chapter Seven - Introduction to Local Area Networ
20
Chapter Seven - Introduction to Local Area Networ
21
Chapter Seven - Introduction to Local Area Networ
Wireless Topology
Not really a specific topology since a workstation in a
wireless LAN can be anywhere as long as it is within
transmitting distance to an access point.
Several versions of IEEE 802.11 standard defines various
forms of wireless LAN connections.
Workstations reside within a basic service set, while multiple
basic service sets create an extended service set.
22
Chapter Seven - Introduction to Local Area Networ
Wireless Topology
Two basic components necessary: the client radio, usually a
PC card with an integrated antenna installed in a laptop or
workstation, and the access point (AP), which is an Ethernet
port plus a transceiver(‫) جهاز اإلرسال واالستقبال‬.
The AP acts as a bridge between the wired and wireless
networks and can perform basic routing functions.
Workstations with client radio cards reside within a basic
service set, while multiple basic service sets create an
extended service set.
23
Chapter Seven - Introduction to Local Area Networ
24
Chapter Seven - Introduction to Local Area Networ
25
Chapter Seven - Introduction to Local Area Networ
Wireless Topology
IEEE 802.11 – The original wireless standard, capable of
transmitting data at 2 Mbps
IEEE 802.11b – The second wireless standard, capable of
transmitting data at 11 Mbps
In actual tests, 11 Mbps 802.11b devices managed 5.5 Mbps
(from a July 2000 test by Network Computing).
26
Chapter Seven - Introduction to Local Area Networ
Wireless Topology
With directional antennae designed for point-to-point
transmission, 802.11b can transmit for more than 10 miles.
With an omni-directional(‫ ) متعددة االتجاهات‬antenna on a typical
AP, range may drop to as little as 100 feet.
27
Chapter Seven - Introduction to Local Area Networ
Wireless Topology
IEEE 802.11a – One of the more recent standards, capable of
transmitting data at 54 Mbps (theoretical) using the 5 GHz
frequency range.
IEEE 802.11g – The other recent standard, also capable of
transmitting data at 54 Mbps (theoretical) but using the same
frequencies as 802.11b (2.4 GHz) and is backwards
compatible with 802.11b.
28
Chapter Seven - Introduction to Local Area Networ
Wireless Topology
HiperLAN/2 (European standard, 54 Mbps in 5 GHz band)
To provide security, most systems use either Wired
Equivalent Privacy (WEP), which provides either 40- or 128bit key protection, or a more advanced standard such as WPA
( Wi-Fi Protected Access ).
Wireless LANs may also be configured without an access
point. These configurations are called “ad-hoc”.
29
Chapter Seven - Introduction to Local Area Networ
30
Chapter Seven - Introduction to Local Area Networ
31
Chapter Seven - Introduction to Local Area Networ
Medium Access Control Protocols
How does a workstation get its data onto the LAN medium?
A medium access control protocol is the software that allows
workstations to “take turns” at transmitting data.
Two basic categories:
1. Contention-based protocols
2. Round robin protocols
32
Chapter Seven - Introduction to Local Area Networ
Contention-Based Protocols - CSMA/CD
Essentially first come first served.
Most common example is carrier sense multiple access with
collision detection (CSMA/CD).
If no one is transmitting, a workstation can transmit.
If someone else is transmitting, the workstation “backs off”
and waits.
33
Chapter Seven - Introduction to Local Area Networ
Contention-Based Protocols - CSMA/CD
If two workstations transmit at the same time, a collision
occurs.
When the two workstations hear the collision, they stop
transmitting immediately.
Each workstation backs off a random amount of time and tries
again.
Hopefully, both workstations do not try again at the exact
same time.
CSMA/CD is half-duplex protocol.
34
Chapter Seven - Introduction to Local Area Networ
Because the number of times a workstation will have to wait is
unknown, it is not possible to determine exactly when a
workstation will be allowed to transmit its data without
collision. Thus, CSMA/CD is a nondeterministic protocol.
35
Chapter Seven - Introduction to Local Area Networ
Contention-Based Protocols - CSMA/CA
CA stands for Collision Avoidance - protocol does not listen
and detect collisions. Instead, it tries to avoid collisions
before they happen
How does CSMA/CA do this?
All devices, before they transmit, must wait an amount of
time called an intra-frame space (IFS)
Some applications have a short IFS, while others have a long
IFS. If two applications want to transmit at the same time,
the application with the shorter IFS will go first.
36
Chapter Seven - Introduction to Local Area Networ
Round Robin Protocols
Each workstation takes a turn transmitting and the turn is
passed around the network from workstation to workstation.
Most common example is token ring LAN in which a
software token is passed from workstation to workstation.
Token ring is an example of a deterministic protocol.
Token ring more complex than CSMA/CD. What happens if
token is lost? Duplicated?
Token ring LANs are losing the battle with CSMA/CD LANs.
37
Chapter Seven - Introduction to Local Area Networ
38
Chapter Seven - Introduction to Local Area Networ
Medium Access Control Sublayer
To better support local area networks, the data link layer of
the OSI model was broken into two sublayers:
1. Logical link control sublayer
2. Medium access control sublayer
Medium access control sublayer defines the frame layout and
is more closely tied to a specific medium at the physical layer.
Thus, when people refer to LANs they often refer to its MAC
sublayer name, such as 10BaseT.
39
Chapter Seven - Introduction to Local Area Networ
40
Chapter Seven - Introduction to Local Area Networ
IEEE 802 Frame Formats
The IEEE 802 suite of protocols defines the frame formats for
CSMA/CD (IEEE 802.3) and token ring (IEEE 802.5).
Each frame format describes how the data package is formed.
The two frames do not have the same layout. If a CSMA/CD
network connects to a token ring network, the frames have to
be converted from one to another.
41
Chapter Seven - Introduction to Local Area Networ
42
Chapter Seven - Introduction to Local Area Networ
Local Area Network Systems
Wired Ethernet or CSMA/CD
Most common form of LAN today.
Star-wired bus is most common topology but bus topology
still not totally dead yet.
Wired Ethernet comes in many forms depending upon
medium used and transmission speed and technology.
43
Chapter Seven - Introduction to Local Area Networ
Wired Ethernet
Originally, CSMA/CD was 10 Mbps
Then 100 Mbps was introduced. Most NICs sold today are
10/100 Mbps
Then 1000 Mbps (1 Gbps) was introduced
10 Gbps is now being installed in high-end applications
44
Chapter Seven - Introduction to Local Area Networ
Wired Ethernet
1000 Mbps introduces a few interesting things:
Transmission is full duplex (separate transmit and receive),
thus no collisions.
Prioritization is possible using 802.1p protocol.
Topology can be star or mesh.
45
46
Chapter Seven - Introduction to Local Area Networ
Wired Ethernet
One of the latest features: Power over Ethernet (PoE)
What if you have a remote device that has an Ethernet
connection? It will require a power connection.
What if you don’t have an electrical outlet nearby?
Use PoE. The power to drive the Ethernet NIC is sent over
the wiring along with the usual Ethernet signals.
47
Chapter Seven - Introduction to Local Area Networ
Wireless Ethernet
As we have already seen, IEEE has created the 802.11b,
802.11a, and 802.11g wireless standards
IEEE 802.11n (100 Mbps) will be ratified soon and should
start appearing in product form in 2007 (maybe?)
Latest wireless Ethernet is using MIMO technology –
multiple input multiple output – sender and receiver have
multiple antennas for optimum reception
48
Chapter Seven - Introduction to Local Area Networ
Local Area Network Systems
IBM Token Ring
Deterministic LAN offered at speeds of 4, 16 and 100 Mbps.
Very good throughput under heavy loads.
More expensive components than CSMA/CD.
Losing ground quickly to CSMA/CD. May be extinct(‫)تنقرض‬
soon.
49
Chapter Seven - Introduction to Local Area Networ
Local Area Network Systems
FDDI (Fiber Distributed Data Interface)
Based on the token ring design using 100 Mbps fiber
connections.
Allows for two concentric (‫ )متحد المركز‬rings - inner ring can
support data travel in opposite direction or work as backup.
Token is attached to the outgoing packet, rather than waiting
for the outgoing packet to circle the entire ring.
50
Chapter Seven - Introduction to Local Area Networ
LANs In Action : A Small Office
Solution
What type of system will interconnect twenty workstations in
one room and fifteen workstations in another room to a
central server, which offers:
• Internal e-mail
• A database that contains all customer information
• High quality printer access
51
Chapter Seven - Introduction to Local Area Networ
52
Chapter Seven - Introduction to Local Area Networ
53
Chapter Seven - Introduction to Local Area Networ
LANs In Action : A Small Office Home
Office (SOHO) LAN Solution
What if you have two computers at home and want both to
share a printer and a connection to the Internet.
Some type of SOHO solution might solve this problem.
Essentially a LAN with a 2- or 3-port hub, connecting cables,
and software.
In some models the hub also acts as a router to the Internet.
54
Chapter Seven - Introduction to Local Area Networ
55